Generated files for tests, Factories etc

[mikeymike]

Work has been done before, https://github.com/AydinHassan/m2-module-import/blob/d9ef47fd51b949381cfb6d32a9e9e37a4d737f0f/test/bootstrap.php

It's likely beneficial to streamline testing when generated files are used so often within the codebase.

Side note, I'm not sure how much of this is already in place in newer versions of Magento and whether this is unit test specific and not also relevent to integration / functional tests.